As a voltage converter, a DC-DC converter which raises and/or lowers a direct-current (DC) voltage has been known. The DC-DC converter has broadly been used in electric devices such as a personal computer, an AV apparatus, a cellular phone and a power supply system each including an electric circuit. In recent years, the DC-DC converter has been used in, for example, power supply systems of vehicles such as a fuel cell car, an electric car and a hybrid car.
The DC-DC converter can be constituted by combining, for example, a switching element such as a transistor, a coil (a reactor), a capacitor, a diode and the like. Examples of the DC-DC converter include a DC-DC converter called a resonant converter which realizes soft switching of the switching element. The soft switching enables a switching operation in a state where a voltage and/or a current is zeroed, by use of a current resonance phenomenon or the like, to decrease power losses at the switching.